1. Field of the Invention
This invention relates to aqueous dispersions containing a silicon-aluminium mixed oxide powder, a process for its production as well as its use for polishing semiconductor substrates.
2. Discussion of the Background
Chemical-mechanical polishing (CMP process) is a technique which is used for planing surfaces and for producing structures in the submicrometer range on semiconductor wafers. Here, a dispersion is used which contains a chemically active compound as well as an abrasive. The latter is of particular importance, as it is to have a high rate of surface removal, without scratching the surface being polished.
It has now become apparent that, depending on the polishing operation involved, it can be useful to employ physical mixtures of abrasive particles and thus to combine the advantages of both components of the mixture.
U.S. Pat. No. 5,891,205 describes a dispersion for use in the CMP process, which contains as an abrasive a physical mixture of cerium oxide and silicon dioxide. Here, the term “physical” means that the dispersion contains particles of cerium oxide and particles of silicon dioxide as separate compounds.
U.S. Pat. No. 5,382,272 likewise describes a physical mixture of cerium oxide particles and silicon dioxide particles; here, an adsorption of the cerium oxide particles onto the silicon dioxide particles is reported to lead to a positive effect during chemical-mechanical polishing. In this connection, a frequent problem is that important variables such as, for example, the particle sizes and the performance within different pH ranges, are not compatible. The result of this is that the anticipated effect is already frustrated at the preliminary stage, if a stable aqueous dispersion cannot be produced from different particles.
The use of chemical mixtures of metal oxides or non-metal oxides has been described in various patents. Thus, for example, U.S. Pat. Nos. 5,858,813 and 5,954,997 describe the use of chemical mixtures of oxides in the CMP process. The chemical mixtures described in these documents, compared with abrasives consisting of only one type of molecule, show no differences with regard to their polishing performance. This means that a specific polishing effect, which would be achieved, for example, using silicon dioxide or aluminium oxide, can alternatively also be attained using the mixed oxides described in these documents.
WO 9905232 A1 describes, in connection with a CMP dispersion, the production of silicon dioxide using a doping agent. It is reported that the hardness of the abrasive particles during the chemical-mechanical polishing process can be varied as a result of the doping. The disadvantage is that the doping agent can be added only within a very narrow range, and so the effect during the chemical-mechanical polishing is only slight.
There is thus a lively interest in the use of mixed oxides in chemical-mechanical polishing. Hitherto, however, there has been no, or only limited, success in demonstrating the advantages of a chemical mixed oxide in chemical-mechanical polishing. This is also to be attributed to the fact that in the patent literature known hitherto, the particles suspended in the dispersion have been described not at all, or only inexactly. Hence consistent reproducible polishing effects cannot be expected either.